An area of space-time with a quantum singularity at its center. The singularity has a theoretically infinite mass, making it impossible for even light to escape. Around the singularity is an Event-Horizon. This is also known as the point of no return because it is the critical area that light can no longer escape from, and time itself comes to an end, or is at the same time ad-infinitum. Much work has been done on this subject by Stephen Hawking.

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A black hole is a point of infinitely dense space with such a high density that no matter or energy may escape. It is formed when a supermassive star supernovaes. It has enough gravity, however, that it will collapse inward on itself. Normally, when this happens, the Palaui Force cannot resist gravity's push (this means that electrons orbiting a nucleus of an atom are pushed inward to the nucleus), and forms neutronium (a superdense form of matter made of only neutrons). Normally, this dense ball of matter forms a neutron star, but in extreme cases, the ball of matter is so dense that it collapses inward on itself. Nothing can stop this collapse once it has started, and it will not end until the matter reaches a point of 0 dimensions (0 feet high, wide, and deep). This forms the singularity of the black hole. There is also a "point of no return", or the event horizon. Once you have crossed this sphere circling the black hole, you will be ripped apart atom by atom, and strung out to an infinite length (called spaghettification). This process will take an infinite amount of time, and nothing will ever get you out. There are also two different types of singularities (that we know about). There are stationary and spinning. In a stationary singularity, the singularity is a single point where the curvature becomes infinite. However, in a spinning singularity, said singularity becomes 2 dimensional, stretching in both space & longitude (this drags space-time surrounding the event horizon, something known as frame dragging). This spinning space-time surrounding the event horizon is known as an ergosphere, which is shaped like an ellipsoid (this is an ellipse, but in 3 dimensions (think about how pluto doesn't orbit the sun right, and you have the idea, represented by the formula {x^2\a^2}+{y^2\b^2}+{z^2\c^2}=1).) This presents a great opportunity for space flight, as a ship grazing the surface of the ergosphere could be launched off at reletavistic speeds. The singularity is most likely a ring shape, instead of a dimensionless sphere. If you fell into a spinning black hole, it is possible you would not spaghettify. If you fell through the inner horizon (the ring singularity) perfectly distanced from all points on the ring, you might make it.
Now, you figure that matter has to go somewhere, and it (theoretically) is ejected at a white hole. A white hole is described as "a postulated celestial body that is the time reversal of a black hole. While a black hole acts as a point mass that attracts and absorbs any nearby matter, a white hole acts as a point mass that repels or (perhaps) even ejects matter." Now, there is no proof of white holes, and it is doubtful you could pass through one (see wormhole, because of a paper published in 1962 by John A. Wheeler and Robert W. Fuller proving that Schwarzschild wormholes are disconnected instantaneously after they are formed, not allowing even light to pass through). I've said waaaaay too much, so I'm going to shut up now. Have a nice day.

Ugh. If you want to see an example, hop on a ship traveling at light speed, wait 3000 years, then jump out. La dee frickin da. Here's the word black hole.

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A single point of infinite density and mass. Blackholes mass is so great that nothing can escape it's gravitational pull, not even light.
Blackholes are supposedly a reality but there is no way (currently) to prove that they are what they are. Blackholes are created by a star or planet of great mass exploding and then collapsing in on itself creating a gravitational pull that just pulls everything in more eventually ending in a blackhole. Blackholes are often identified by circles of light that orbit them because of the gravity. Blackholes also give off X-Ray and infared signals making them easier to discover.
Blackholes are often theorized as tears in the time-space continuum meaning they can be portals to other points in space or time. Many sci-fi books and movies theorize blackhole possibilities, such as L. Ron Hubbard's "Mission Earth" novels using blackholes as an energy source.

Bob: If a blackhole existed on Earth for more than 3 seconds, Earth would be gone.
Joe: But I hear scientists are attempting to create bla-- *Earth is sucked into blackhole in 3 seconds*